Several different types of passive samplers are known in the art for measuring relative abundance of volatile organic compound (VOC) vapors below the ground surface, but the uptake rate of the sampler and the delivery rate of vapors to the void space in which the sampler is deployed has not been known or controlled, so the ability to quantify vapor concentrations from the mass of compounds sorbed was not very accurate or precise. As a result, they have been labeled as useful only for screening purposes and not for purposes requiring higher levels of data validation such as human health risk assessment.
Prior passive soil vapor sampling devices include the Gore® Sorber or Gore® Module, Petrex Tubes, Beacon's B-Sure Test Kit, and EMFLUX cartridges. Each of these consists of sorbent media exposed to a void space in the subsurface with no rigorous attempt to maintain an uptake rate by the sampler that is lower than the delivery rate of vapors from the soil. In this case, the uptake rate varies according to the porosity and moisture content of the soil, which is seldom consistent.
For some existing passive soil vapor samplers, concentrations can at best be estimated by deriving an empirical relationship between the mass sorbed and concentrations measured with conventional active soil gas sampling methods; however, this has limitations because the empirical relationship depends on site-specific conditions, which may vary unpredictably between sites and sampling locations. Others (e.g., the Gore™ Module) have attempted to use an equation to calculate the uptake rate from the soil moisture and porosity, but they did not derive their equations from first principles, and have not shown their approach provides accurate and precise concentration data.
What is needed is a sampler that constrains the uptake rate of the sampler to be lower than the delivery rate for most commonly encountered conditions of soil porosity and moisture content, to provide consistent quantification of soil vapor concentrations. The uptake rate must also be high enough to allow the sampler to detect low soil vapor concentrations with a practical sampling duration.